Prove that v is an eigenvector of operator B

[Smazmbazm]

Homework Statement

Let B be the linear operator

(1-x^{2}) \frac{d^2}{dx^2}-x\frac{d}{dx}

Show that

T_{4}(x) = 8x^{4} - 8x^{2} + 1

is an eigenvector of B, and find the corresponding eigenvalue.

Attempt

Righto, I find these rather difficult so a step by step solution would be nice but I know that's a lot of typing and formatting so I'm grateful for any corrections.

Step 1: Bv_{4} = \lambda _{4} v_{4}

Step 2: ((1-x^{2}) \frac{d^2}{dx^2}-x\frac{d}{dx})(8x^{4} - 8x^{2} + 1) = \lambda _{4} (8x^{4} - 8x^{2} + 1)

Step 3: This is the first point where I'm unsure of what comes next. Should I be doing something like this

\frac{d^2}{dx^2}((1-x^{2})(8x^{4} - 8x^{2} + 1)) - x\frac{d}{dx}(8x^{4} - 8x^{2} + 1) = 8\lambda _{4}x^{4} - 8\lambda _{4}x^{2} + \lambda _{4}<br /> <br /> \\ \frac{d^2}{dx^2}(-8 x^6 +16 x^4 -9 x^2 + 1) - \frac{d}{dx}(8x^{5} - 8x^{3} + x) = 8\lambda _{4}x^{4} - 8\lambda _{4}x^{2} + \lambda _{4}<br /> <br /> \\ -240x^4 + 192x^2 - 18 - 40x^4 + 18x^2 - 1 = 8\lambda _{4}x^{4} - 8\lambda _{4}x^{2} + \lambda _{4}<br /> <br /> \\ -280x^4 + 216x^2 - 19 = 8\lambda _{4}x^{4} - 8\lambda _{4}x^{2} + \lambda _{4}

Is this all correct so far? If so, awesome. Now I don't really know how to go about solving for \lambda

In general, is it enough to say that \lambda = \frac{-280x^4 + 216x^2 - 19}{8x^{4}- 8x^{2} + 1}

 

[CompuChip]

Step 3: This is the first point where I'm unsure of what comes next. Should I be doing something like this

= 8\lambda _{4}x^{4} - 8\lambda _{4}x^{2} + \lambda _{4}<br />

<br /> <br /> You can do that, but you don&#039;t need the right hand side. It&#039;s probably easier to start by calculating <br /> $$B v = \frac{d^2}{dx^2}((1-x^{2})(8x^{4} - 8x^{2} + 1)) - x\frac{d}{dx}(8x^{4} - 8x^{2} + 1)$$<br /> and after simplifying you should get a multiple of $8x^{4} - 8x^{2} + 1$ from which you can read off the value of $\lambda$.<br /> <br /> <blockquote data-attributes="" data-quote="" data-source="" class="bbCodeBlock bbCodeBlock--expandable bbCodeBlock--quote js-expandWatch"> <div class="bbCodeBlock-content"> <div class="bbCodeBlock-expandContent js-expandContent "> $$ \frac{d^2}{dx^2}(-8 x^6 +16 x^4 -9 x^2 + 1) - \frac{d}{dx}(8x^{5} - 8x^{3} + x) $$ </div> </div> </blockquote><b>No!</b> You cannot move the x after the derivative operator:<br /> $$x \frac{d}{dx} x^n \neq \frac{d}{dx} x^{n+1}$$

 

[Smazmbazm]

Ok great, thanks for the response. By that logic, wouldn't it imply that you can't move the (1-x^2) to the right side either? So you just take the double derivative of the eigenvector then multiply by (1-x^2) minus the first derivative of the eigenvector multiplied by x then solve for lambda. If I follow that process, I get a lambda of -16 =] so I guess that's the way to do it